An airbag system, which is occupant restraint system, has been widely adopted in recent years for improving safety of the occupants in a vehicle. The airbag system operates on the principle that a gas generator is operated under control of signals from a sensor detecting a collision, to inflate an airbag between occupant and a car body. The gas generator is required to have a function to produce a required and sufficient amount of clean gas containing no harmful gas in a short time. On the other hand, the gas generating agent is press-formed into a tablet form for stability to the combustion, and the transfer charge is formed into a granule form for use. The tablets and granules are required to maintain their initial combustion characteristics over a long time even under various harsh environments. In the event that the tablets deforms or decreases in strength due to deterioration with age, change of environments and the like, the combustibility of the explosive composition will exhibit at an abnormally earlier time than the initial combustibility, so there is a fear that the airbag or the gas generator itself may be broken with the abnormal combustion in case of a collision, to fail in accomplishing the aim of protecting the occupants or even cause them injury.
To satisfy those required functions, gas generating agents containing metallic compound azide such as sodium azide and potassium azide as their major ingredient have been used hitherto. These known gas generating agents are widely used in terms of their advantages that they are burnt momentarily; that the ingredient of combustion gas is substantially nitrogen gas only, so that no harmful gas such as CO (carbon monoxide) or NOx (Nitrogen oxide) is produced; and that since the combustion velocity is little influenced by the environment or the structure of the gas generator, it is easy to design the gas generator. However, these known gas generating agents have a disadvantage to be readily exploded by impact and friction, so it is difficult to make them explosion-proof, as demonstrated by large and small explosion accidents happened here and there in the manufacturing process. Further, the known gas generating agents have a notable disadvantage that they decompose in the presence of water and acid then produce a harmful gas. Due to this, it comes to be urgently necessary these days to develop a safer gas generating agent and put it into practical use, in substitution for the known gas generating agents whose major ingredient is the metallic compound azide.
On the other hand, the method in which tetrazoles including amino tetrazole are mixed with and used in combination with the metallic compound azide has been proposed in, for example, Japanese Laid-open Patent Publications No. Sho 49(1974)-87583, No. Hei 2(1990)-184590 and No. Hei 2(1990)-221179. Since molecules of the tetrazoles have a high proportion of atoms of nitrogen such that production of CO can be suppressed, almost no CO is produced in the combustion gas, as in the case with the metallic compound azide. Besides, the tetrazoles are superior to the above said metallic compound azide in far less danger and toxicity. The gas generating agent of this type comprising the mixture of the tetrazoles with the metallic compound azide succeeded in lessening the problems involved in the gas generating agent containing the metallic compound azide as its major ingredient, as compared with the one singly using the metallic compound azide, but has not yet succeeded in solving the above said problems fundamentally, as long as its using the metallic compound azide.
Accordingly, in order to make the best use of the advantages of the tetrazoles, a modified method using tetrazoles singly rather than in combination with the metallic compound azide was proposed, as disclosed in, for example, Japanese Patent Publications No. Sho 64(1989)-6156 and No. Sho 64(1989)-6157 and Japanese Laid-open Patent Publications No. Hei 2(1990)-225159, No. Hei 2(1990)-225389, No. Hei 3(1991)-20888, No. Hei 5(1993)-213687, No. Hei 6(1994)-80492, No. Hei 6(1994)-239684 and No. Hei 6(1994)-298587. The method using the tetrazoles containing no hydrogen (JP Patent Publications No. Sho 64(1989)-6156 and No. Sho 64(1989)-6157 and JP Laid-open Patent Publications No. Hei 6(1994)-80492 and No. Hei 6(1994)-239684) is, in particular, superior in that moisture is not contained In the produced gas. The moisture may condense in the airbag to sharply decrease the volume. However, this method has a disadvantage that the tetrazoles themselves were low in combustibility, so that the tetrazoles used as the gas generating agent often interrupted the combustion then hinder the complete combustion of the gas generating agent.
Accordingly, for improvement of the combustibility, a retrospective method of a combined use of the tetrazoles and the metallic compound azide (the above said JP Laid-open Patent Publication No. Hei 2(1990)-221179) and a method using a powerful oxidizing agent such as chlorate or perchlorate (the above said JP Laid-open Patent Publication No. Hei 6(1994)-298587) were proposed. However, the former had the above said safety problem inherent in the metallic compound azide, while the latter had the problem that despite of using tetrazoles of higher level of safety, the safety is resultantly reduced by the use of the powerful oxidizing agent. In addition, when chlorate or perchlorate was used as the oxidizing agent, there are another problem that combustion temperature rose and resultantly NOx was generated.
The generation of NOx may be restrained by using low-combustibility nitrates or nitrites as the oxidizing agent, but in this case, since the nitrate and nitrite have the property of absorbing heat then decompose during the reaction of the oxidizing agent with the tetrazoles, their inherent drawbacks of poor ignitability and slow combustion velocity are amplified, so that the above said grave problem, that the gas generating agent once ignited cannot lead to a complete combustion, remains still unsolved.
Further, in the system using a powerful oxidizing agent such as chlorate or perchlorate, there was presented a serious drawback that a pressure exponent of combustion reaction is so high that the combustion must be controlled with difficulties.
Specifically, the relation between the combustion velocity (dW/dt) and the pressure in combustion of explosive is expressed by the following formula: EQU dW/dt=A.multidot.P.sup.n Formula (5)
where W represents an explosive combustion amount (g), t represents time (second), A represents a constant by the system, P represents a pressure (atm), and n represents a pressure exponent (a constant by the system).
On the other hand, the relation between the velocity (dWG/dt) for discharging the gas from the gas generator and the pressure is expressed by the following formula: EQU dW.sub.G /dt=K.multidot.P.sup.0.5 Formula (6)
where WG represents an amount (g) of discharging the gas from the gas generator, t represents time (second), K represents a constant by the system, and P represents a pressure (atm.).
It is understood from the formulas (5) and (6) that since the combustion velocity of the gas generating agent is proportional to the power of nth of the pressure P and the velocity of discharging the gas from the gas generator is proportional to the power of 0.5th of the pressure P, if the pressure exponent n is more than 0.5, the combustion amount becomes more than the amount of discharging the gas from the gas generator, so that the pressure in the gas generator comes to rise gradually. Here, if the pressure exponent n is remarkably large, the pressure in the gas generator will rise sharply to cause the combustion velocity to increase more and in turn cause the pressure in the gas generator to rise more and more, which will eventually cause an explosion of the container. The above said method using a powerful oxidizing agent such as chlorate or perchlorate (the above said JP Patent Laid-open Publication No. Hei 6(1994)-298587 and others) had the problem of the pressure exponent becoming too large to control the combustion. Further, it is known that the metallic compound azide allows an easily filterable slag to be formed by its combined use with silicon dioxide, but, disadvantageously, using the tetrazoles makes it difficult to form the easily filterable slag.
With these non-azide base gas generating agent compositions, the fuel ingredients are the above said organic compounds including the tetrazoles mentioned above, whereas the oxidizing agents are inorganic compounds including chlorate or perchlorate. Due to this, there arises a problem in formability of tablets and the like when a usual binder is used, so that the non-azide base gas generating agents formed into tablets and the like were rather inferior in mechanical strength to those of the azide base gas generating agents. Also, in the thermal shock tests in which environmental temperature around the tablets and the like are raised and fallen repeatedly, it was found that due to difference in coefficient of thermal expansion between the orgahic compound and the inorganic compound, the binding power of the binder decreased gradually, and there were some extreme cases where the forms deteriorated into powder. Accordingly, JP Laid-open Patent Publication No. Hei 6(1994)-219882 proposed that combustible polymers including polyurethane, cellulose acetate, hydroxy-terminated polybutadiene and ethyl cellulose are used as the binder. However, when these organic polymeric compounds are used, there arises a problem of increasing concentration of harmful carbon monoxide (CO) in the combustion gas together with a calorific value, which in turn arises the need for increasing an amount of a cooling material (a woven metal wire or equivalent) for cooling the generated gas. As a result, the gas generator increases in size and weight against the times' demands of reduction of size and weight of the system.
Also, what is called "a boron niter" having boron and potassium nitrate as its major ingredients is generally used as an enhancer charge for igniting the gas generating agent. However, no matter which of the metallic compound azides and the tetrazoles is used as the gas generating agent, since the boron niter is quite different from either of them in composition, the enhancer must be disadvantageously produced in a separate process independent of the production process of the gas generating agent.
The present invention aims to solve the above said problems involved in the known airbag explosive composition including the above-mentioned known gas generating agent and enhancer. Specifically, the present invention provides a novel airbag explosive composition capable of providing good formability even when the gas generating agent has organic nitrogen containing compound as its ingredient; good combustibility with solving the problems involved in the conventional type gas generating agents having the metallic compound azides or the tetrazoles as their major ingredient; high safety with the best possible use of the advantages of the tetrazoles; easy combustion controllability; and high in slag forming ability. In other words, the objects of the present invention are:
(1) to provide a novel binder which can provide good formability and properties in the presence of an inorganic oxidizing agent even if the fuel ingredients are the tetrazoles or other organic nitrogen containing compounds; PA1 (2) to provide an explosive composition which is easy to handle and high in safety without generating any harmful gases; PA1 (3) to provide an explosive composition which is low in pressure exponent or easy to control of the combustion, even in the combination of the tetrazoles and the powerful oxidizing agent such as chlorate or perchlorate; PA1 (4) to provide a novel explosive composition which can provide improved combustibility to allow the complete combustion of the explosive composition, even in the combination of the tetrazoles and the oxidizing agent of poor combustibility such as nitrate or nitrite; PA1 (5) to provide a novel explosive composition which can form an easily filterable slag to obtain a clean gas; and PA1 (6) to provide a novel explosive composition which is identical in composition to the gas generating agent and usable as the enhancer as well. PA1 M.sup.2+ represents a bivalent metal such as Mg.sup.2+, Mn.sup.2+, Fe.sup.2+, Co.sup.2+, Ni.sup.2+, CU.sup.2+ and Zn.sup.2+ ; PA1 M.sup.3+ represents a trivalent metal such as A1.sup.3 +, Fe.sup.3+, Cr.sup.3+, Co.sup.3+ and In.sup.3+ ; PA1 A.sup.n- represents an n-valence anion such as OH.sup.-, F.sup.-, Cl.sup.-, NO.sub.3.sup.-, CO.sub.3.sup.2-, SO.sub.4.sup.2-, Fe(CN).sub.6.sup.3-, CH.sub.3 COOH.sup.-, oxalate ion and salicylate ion; and PA1 0&lt;x.ltoreq.0.33. PA1 1 tetrazole group including one or more hydrogen atoms; PA1 2 aminotetrazole group other than 1; and PA1 3 an alkali metal salt, an alkali earth metal salt or an ammonium salt of the above said 1 or 2. PA1 4: one or more kinds of zirconium, hafnium, molybdenum, tungsten, manganese, nickel, iron or the oxide or sulfide; and PA1 5: one or more kinds of carbon, sulfur or phosphorus.